Here's an analogy to the problem I wish to solve:

Say I have three cells, A1, B1, C1.

In A1 is a static value or list of values, say 5; in B1 is a function, say A1^2, which would equal 25. In C1, I want to write a formula that would give me the output of B1, if A1 were equal to say, 10. So I'd enter 10 into C1, and I'd get 100. But A1 would still have 5 in it.

So
A1 = 5
B1 = A1^2;
C1 = Value of B1 if A1 were made to equal [user input in C1]

Breaking up C1 into two cells, eg user input and function, would work too. So C1 = 10, and D1 = Value of B1 if A1 were made to equal C1.

I have a spreadsheet model which computes outputs based on a sequence of steps. There are several possible inputs into the model, and several outputs. As it is, if I want to get outputs, I have to manually copy paste the outputs and make them static, rather than have outputs dynamically displayed for all possible inputs.